Microstructural evolution of new type Al-Zn-Mg-Cu alloy with Er and Zr additions during homogenization

A comprehensive study on the microstructural evolution of a new type Al-Zn-Mg-Cu-Er-Zr alloy duringhomogenization was conducted by optical microscope,scanning electron microscope,transmission electron microscopy and X-raydiffraction analysis.The results show that serious segregation exists in as-cast alloy,and the primary phases are T(AlZnMgCu),S(Al2CuMg)and Al8Cu4Er,which preferentially locate in the grain boundary regions.The soluble T(AlZnMgCu)and S(Al2CuMg)phases dissolve into the matrix gradually during single-stage homogenized at465°C with prolonging holding time,but the residualAl8Cu4Er phase cannot dissolve completely.Compared with the single-stage homogenization,both a finer particle size and a highervolume fraction of L12-structured Al3(Er,Zr)dispersoids can be obtained in the two-stage homogenization process.A suitablehomogenization scheme for the present alloy is(400°C,10h)+(465°C,24h),which is consistent with the results of homogenizationkinetic analysis.

Mechanical properties and corrosion behavior of a new RRA-treated Al-Zn-Mg-Cu-Er-Zr alloy

The effect of various retrogression and re-aging(RRA)treatments on mechanical properties and corrosion behavior of a new type Al-Zn-Mg-Cu-Er-Zr alloy was investigated by microhardness testing,tensile testing,intergranular corrosion(IGC)and exfoliation corrosion(EXCO)testing.The results show that the RRA treatment can effectively improve the IGC and EXCO resistance with less strength sacrificing because the grain interior precipitates and the grain boundary precipitates are similar to that of T6 temper and T73 temper.Meanwhile,as the microhardness of the retrogressed alloy reaches the peak value during the retrogression at 170℃,the corresponding RRA-treated alloy possesses the better strength,but the corrosion resistance is poor in comparison with that of the retrogression at 190℃.The optimal combination of strength,IGC and EXCO resistance is obtained after retrogression at 180℃for 60 min.Moreover,for different heat treatment tempers,the corresponding micros true tural evolution of the precipitates including Al_(3)(Er,Zr)particles was also discussed in detail.